1. Field of the Invention
This invention relates to a cable anchorage, particularly for anchoring a submarine cable to a submersible housing such as a splice chamber or the like, but not exclusively so.
2. Discussion of the Prior Art.
Pat. application WO 8704573 discloses a cable termination suitable for a submarine telecommunications cable. It teaches that where a submarine telecommunication cable is concerned, tensile load-bearing elements are present as one or more annular layers of wires, wound helically along the cable. For example, shallow water telecommunications cables are generally provided with armour wires for protection against damage by trawlers. These armour wires between them may have to take tensile loads of the order of 100 tons if a trawler should pick up a cable in error. At a repeater housing loads of up to for instance 3 tons may have to be borne by the individual wires as the repeater housing is wound onto a cable drum. Hence any clamp assembly used in clamping the ends of the armour wires, known in this context as an armour clamp, must be capable of reliably withstanding total loads on the wires of the order of 100 tons and, if used at a repeater housing, of more than 3 tons on individual wires.
It goes on to describe prior art armour clamps and their problems as follows. It describes a known type of armour clamp comprising a pair of conical rings. The outer ring presents a conical hole, complementary to the exterior of the inner ring. In use, the ends of the armour wires are each fed through the inner ring and bent back over its conical exterior, round the thicker end of its walls. The outer ring is seated on the wires, its conical hole clamping them against the exterior of the inner ring. The outer ring can then be appropriately mounted to anchor the cable as required, for example, to a repeater housing.
This assembly is self-tightening in use. The surface of the conical hole of the outer ring is profiled so as to grip the armour wires while the exterior of the inner ring is smooth. Any tensile load on the armour wires then acts to pull the outer ring towards the thicker end of the inner ring and so tends to increase the clamping effect on the wires themselves, between the two rings. However, unfortunately, the assembly has been found unsatisfactory. The bend produced in each armour wire has led to failure of the clamp owing to shear breakage under unacceptably low tensile loads.
It explains that in order to overcome the problem recognised in the above assembly, a second known type of armour clamp designed to operate without requiring the armour wires to be bent, is described in British Pat. GB No. 2122245. In this assembly, a rigid sleeve is inserted under the straight ends of the armour wires. Coaxial rings having complementary conical surfaces are again used but, in this case, both conical surfaces are smooth and the inner ring is in four separate segments, divided along radial planes of the ring.
The segments of the inner ring are mounted on the armour wires over the rigid sleeve, and the outer ring is then mounted over the segments.
In this second type of armour clamp, which is again self-tightening, when tensile load is applied to the armour wires the tow rings are loaded such that the inner ring is pulled towards the end of the outer ring having the narrower opening. The segments of the inner ring are thereby clamped more tightly onto the armour wires, against the rigid internal sleeve.
Although the second type of armour clamp has been found to offer acceptable reliability at loads of up to 82 tons on the armour wires, it also has drawbacks, including cost, owing to the number of components involved, and the need to insert the rigid sleeve under the straight ends of the armour wires. This means that the diameter of parts of the cable which lie within the armour wires must be reduced to give access for the rigid sleeve.
A further problem, common to known forms of armour clamp, particularly for submarine cables, is related to the techniques required for their installation.
Conventionally, such clamps are mounted, and the clamping forces provided, by a circumferential ring of bolts. Initially, the practice is to tighten these bolts evenly to a controlled maximum tension, selected according to the loads expected in use. The clamp components are thereby settled-in and the likely extent of any movements in the clamp components when in use is consequently reduced.
However, it is known that to maintain the cable armour under such maximal stress can lead to accelerated local deterioration of the armour, particularly from salt water corrosion for example. Therefore, the tension is normally relieved by subsequently loosening the bolts and then tightening them to a final installation tension slightly lower than the original maximum applied tension. This two-stage mounting process, requiring balanced tightening of the bolts using a torque wrench, is clearly laborious and time-consuming.
The clamp assembly which is the subject of the aforementioned WO 8704573 has advantages over the prior art referred to but the preferred embodiment described also has a drawback. The roughening comprises circumferential corrugations on the outer surface at the first ring. If the wires are high tensile steel wires then they are of a relatively hard steel. Such wires are used for the central strength member of the submarine cable such as is described in our British Pat. No. 1550588, whereas any additional armour wires applied on the outside of the cable will normally be of lower strength material such as mild steel (but of larger overall cross section and therefore of substandard strength).
Where it is desired to clamp to high tensile wires then the corrugations need to be of a harder material than the wires, otherwise the wires will `blunt` the corrugations, thereby substantially reducing the grip of the clamp assembly. It is possible to harden the corrugations by, e.g. a case-hardening technique, but this can be unreliable, and the hardened points can crack under load.